This invention is generally directed to a process for rapidly charging uncharged toner particles to a positive polarity, and more specifically, to a process for rapidly charging uncharged toner particles added to a charged developing composition by employing in the process polymer coated carrier particles containing thereon or therein certain specific additives for the purpose of increasing the rate at which the uncharged toner particles are charged to a positive polarity.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known, particularly wherein the latent image is developed by applying toner particles thereto using for example, the cascade development method as described in U.S. Pat. No. 3,618,552, magnetic brush development as disclosed in U.S. Pat. Nos. 2,874,063, and 3,251,706, or touchdown development, reference U.S. Pat. No. 3,166,432. Also known is the production of a reverse copy of an original, thus for example, a negative copy can be produced from a positive original, or positive copy can be obtained from a negative original.
In these processes, especially in cascade development, generally a developer composition comprised of relatively large carrier particles containing finely divided toner particles electrostatically clinging to the surface of the carrier particles is conveyed to and rolled or cascaded across the surface bearing the electrostatic latent image. Negatively charged electrostatic latent images are developed by utilizing a toner and carrier combination wherein the tone is triboelectrically charged positive in relation to the carrier, while positively charged electrostatic images are developed utilizing a toner and carrier combination wherein the toner is triboelectrically charged negative in relation to the carrier material. The triboelectric relationship between the toner and carrier particles is primarily dependent on the relative position of these materials in the triboelectric series, wherein they are arranged in ascending order of their ability to assume a positive charge. Thus for example, each material is positive in this series with respect to any material classified below it, and negative with respect to any material classified above it. As the developer mixture cascades or rolls across the image bearing surface, the charged toner particles are electrostatically attracted to the charged portions of the image bearing member, whereas they are not electrostatically attracted to the uncharged or background portions of the image subsequent to development. Subsequently, the unused toner and carrier particles can be recycled into the developer supply reservoir.
In another form of development known as magnetic brush development, a developer material comprised of toner particles and magnetic carrier particles is transported by a magnet, whereby the magnetic field of the magnet causes alignment of the magnetic carriers, in relation to the electrostatic latent image bearing surface, causing the toner particles to be attracted to the electrostatic image by electrostatic attraction, reference U.S. Pat. No. 2,874,063.
The use of certain charge control agents for the purpose of imparting a positive charge to the toner particles contained in the developer composition is also known. Thus for example, the use of quaternary ammonium salts as charge control agents for electrostatic toner compositions is disclosed in U.S. Pat. No. 3,893,935. In accordance with the disclosure of this patent, certain quaternary ammonium salts when incorporated into toner materials were found to provide a toner composition which exhibited relatively high uniform and stable net toner charge, when mixed with a suitable carrier particle. U.S. Pat. No. 4,079,014 contains a similar teaching with the exception that a different charge control agent is used, namely a diazo type compound.
Carrier materials utilized in the developer mixture, and in the development of electrostatic images are described in many patents including U.S. Pat. No. 3,590,000, the type of material used being dependent for example on a number of factors, such as the type of development process employed, the quality of image development desired, the composition of the imaging member, and the like. In many instances, the carrier materials utilized should have a triboelectric value commensurate with the triboelectric value of the toner particles, in order to enable electrostatic adhesion of the toner particles to the carrier particles. Additionally, carrier particles which have carrier coatings thereon should preferably be comprised of coatings having a certain hardness, primarily for durability purposes, however, the materials should not be of a hardness that will scratch the imaging member surface, upon which the electrostatic image is initially placed. Carrier particles should also be selected that are not brittle so as to cause flaking of their surface, or particle breakup under the forces exerted on the carrier during use, as this will cause undesirable effects since, for example, the flakes produced may be transferred to the imaging surface thereby reducing the quality and readability of the final image.
Recent efforts have been concerned with polymer carrier coatings that can be recycled, and do not cause any harmful undesirable effects to the imaging surface. However, many of the coatings utilized deteriorate rapidly particularly when employed in a continuous process, and in some instances the entire coating separates from the carrier core in the form of chips or flakes, which may be caused by poor adhesion of the coating material that fails upon contact with machine parts, and other carrier particles. Also the triboelectric values of some carrier coatings fluctuate when changes in relative humidity occur, thus such carriers are not desirable for use in electrostatic systems since they adversely affect the quality of the resulting image.
Many of the above described developers, have a tendency to lose their charge over an extended period of time, and in some instances charge control additives employed to reduce this loss are incompatible with the thermoplastic resin, thus making it difficult to uniformly disperse or dissolve such materials in the toner. Additionally, when new uncharged replenishment toner is added to the charged developer composition that is contained in the electrostatographic system, it does not acquire the desired positive charge until a substantial period of mixing has been accomplished. It is important in order to obtain high quality images on a continual basis, as well as for other purposes, that the new replenishment toner being added acquires rapidly the appropriate charge. Also some of the additives, such as the charge control agents of U.S. Pat. No. 3,893,935 are soluble in water, causing them to be leached to the toner surface by moisture, thereby adversely affecting the machine environment and copy quality.
New replenishment toner particles being added to a developer composition contained in a commercial electrophotographic device has essentially a neutral average charge. In order for the toner to properly cause development of an electrostatic latent image, it must be charged to an appropriate level, which generally involves substantial mixing of the replenishment toner particles with the developer composition for a period of time equal to or greater than 15 minutes. Such a time delay is undesirable as the replenishment toner particles can be printed out as background, and the electrical properties of the developer composition are adversely affected, to the extent that images of low quality and low resolution result, and in some instances development does not occur. Also, replenishment toner particles not charged to the appropriate polarity and magnitude can cause contamination as a result of the deposition of such particles on machine parts, thereby eventually, for example, causing the failure of charging corotrons, filters, and the like. Such failures are not only costly, but result in images of low quality, or no images whatsoever.